Duane syndrome (DS) is a common form of congenital cranial dysinnervation disorders. The ocular motility pattern lies in a wide clinical spectrum, and the choice of treatment must be individualized depending on the severity of the clinical findings. There is no perfect method of treatment and no real “cure” in DS. In this paper, the aim is to give some guidelines to the reader for selection of the most appropriate treatment method for the patient.

In its original description, Duane syndrome (DS) is defined as an ocular motility disorder characterized with severe abduction deficiency, variable limitation of adduction, globe retraction with narrowing of the palpebral fissure, and oblique elevation or depression on adduction.[1] DS represents 1%–4% of strabismic population; it is more common in females and more common in left eyes.[2] Bilaterality is not uncommon, and nearly 10% of the cases are reported to be familial. In this paper, factors about etiology that may influence surgical results, goals of treatment, and surgical treatment methods with possible results will be reviewed with a decision-making approach.

Etiology and Classification

DS is primarily an innervational disorder with secondary restrictive problems. Electromyographic (EMG) studies revealed paradoxical contraction of lateral rectus (LR) muscle on adduction.[3],[4],[5] Histopathological studies demonstrated the absence of abducens nucleus and partial innervation of LR muscle by branches of oculomotor nerve.[6],[7],[8] The number of the cases are limited in histopathological studies. The studies with neuroimaging devices provided information in a large group of patients with broad clinical spectrum and led us to understand the variable manifestations of the abnormal development of ocular motor nerves.[9],[10],[11] In an early brainstem magnetic resonance imaging (MRI) study, we could only demonstrate the abducens nerve on the normal side of two out of ten patients.[12] However, with new-generation MRI equipment, more reliable data could be obtained in various types of DS. Kim and Hwang [13] demonstrated that the abducens nerve was absent in all cases with Type 1 DS, it was present in all Type 2 patients, and it was either absent or present in Type 3 DS. Studies with MRI also demonstrated that the LR muscle thickness was within normal range as the problem is a dysinnervation, not a “lack” of innervation.[12],[14],[15] The absence of denervation atrophy may serve as a clue for the differential diagnosis of DS with abducens nerve palsy in cases where there is any confusion.

Familial cases suggested an underlying genetic problem, and the gene mapping is done in familial isolated DS.[16] The associated abnormalities and the association of DS with thalidomide embryopathy suggested a teratogenic effect during second gestational month. However, no common maternal factor could be demonstrated up to date in isolated DS.[17] DS is accepted as the most common form of “congenital cranial dysinnervation syndromes” (CCDD).

In Huber's [2] well-known classification, DS is divided into three types. The features of these three types may be summarized as below:

In typical forms, the main EMG abnormality is paradoxical contraction of LR muscle. However, paradoxical contraction of medial rectus (MR), inferior oblique (IO), and vertical rectus muscles was previously reported in typical forms, but the abnormal contraction of these muscles is usually underestimated.[3],[4],[5],[18]

The abnormal innervation of extraocular muscles which does not fit Huber's classification is called as “atypical DS.”

Vertical retraction syndrome is one of these described forms. These patients may have the features of a horizontal DS plus a globe retraction on vertical positions of gaze with or without limitation of vertical eye movements. In a case with Type 1 DS plus globe retraction on adduction, elevation, and depression, we performed EMG [Figure 1]. That case demonstrated paradoxical contraction of vertical recti on elevation, depression, adduction, and on attempted abduction. Scott and Wong [4] demonstrated abnormal activity of superior rectus (SR), inferior rectus (IR), and IO muscles on adduction. However, in the presented case, both vertical recti contracted not only on adduction but also on abduction, elevation, and depression.

Figure 1: Left type 1 Duane syndrome with globe retraction on adduction, upgaze and downgaze. Both vertical rectus muscles were found to have contraction with electromyography on elevation, depression adduction, and abduction

Synergistic divergence is another form of DS that is characterized with divergence on abduction and limitation of adduction. EMG data demonstrated co-contraction and excessive LR firing on adduction.[19] The unopposed paradoxical contraction of LR with an underacting MR is claimed for this peculiar motility disorder. We reported a group of three patients with synergistic divergence in association with congenital fibrosis of extraocular muscles (CFEOM).[20] Another group of four patients with CFEOM and synergistic divergence is also reported.[21] The fibrotic changes appear secondary to innervational abnormalities both in CFEOM and DS, so it is suggested that these two ocular motility disorders are varieties of the same clinical spectrum as CCDD.

Another form of atypical DS is Y-pattern deviation. Kushner [22] demonstrated paradoxical contraction of LR muscle in Y-pattern deviation and proposed that it was a variant of DS. We reported a group of four patients with Y-pattern deviation, and our results also suggested that it was a variant of DS.[23] However, our results demonstrated that the co-contracting muscle might be the IR muscle as well as the LR muscle.[23],[24] The paradoxical contraction of IR rectus muscle was demonstrated either by EMG or kinematic MRI examination.

It is considered that the up and downshoots in adduction are secondary to the leash effect of paradoxically contracting fibrotic LR muscle.[25] We suggested that the mechanism of a Y-pattern with a co-contracting IR muscle might be similar with the mechanism of upshoot on adduction.[24] If a co-contracting LR muscle may cause an upshoot on adduction, then a co-contracting IR muscle may cause abduction on elevation – a similar condition that is 90° turned in clockwise.

In an attempt to demonstrate some common clues among atypical forms of DS, we reported a group of nine patients with atypical forms of DS.[26] There was no globe retraction in four patients, and paradoxical contraction of vertical rectus muscles was found in three of the cases. The findings of our patients suggested that globe retraction is not a common abnormality in atypical forms of DS. It was previously demonstrated that globe retraction may not occur even in typical forms of DS.[27] We concluded that the term retraction syndrome needs to be abandoned. We proposed the addition of atypical forms into Huber's classification as below:[26]

Type 4: Synergistic divergence

Type 5: Vertical retraction syndrome

Type 6: Y-pattern deviation.

Even more atypical forms may be observed in DS. We previously reported a case with λ-pattern exodeviation and globe retraction on adduction.[28] In this case, kinematic MRI examination demonstrated bilateral accessory extraocular muscle below the optic nerve that demonstrated a paradoxical contraction on downgaze. This case also had bilateral IR hypoplasia and was the first documented case with paradoxical contraction in an accessory extraocular muscle.

Although Huber's classification does not perfectly cover all of the cases and there are overlaps, especially in Type 1 and Type 3 cases, it is still useful for an easy documentation of the cases with DS.

Common Clinical Features

The major clinical finding is a limitation of abduction and/or adduction that may be associated with esotropia, exotropia, or orthophoria. Abnormal head posture and globe retraction on adduction are the other common clinical features. Up and downshoots on adduction and alphabetical patterns are also frequent. Children with DS require regular follow-up as amblyopia is a frequent finding. Binocular functions are usually good in most of the patients as many of them have orthophoria in a certain position of gaze with abnormal head posture.

Treatment

The surgeon must be ready for surprises as DS is a dysinnervation plus restriction and the extraocular muscles do not behave as in concomitant strabismus. The general principle is not to operate unless clear-cut indications are present. As a general rule, resections should be avoided, and the normal eye may be operated where necessary. As the predictability of surgical outcome is low compared to conventional strabismus surgery, adjustable sutures are preferable where possible.

The surgeon must be aware that in both typical and atypical forms of DS, there may be a paradoxical contraction in extraocular muscles other than LR muscle, and this paradoxical contraction may even affect accessory extraocular muscle (s).

The goals of treatment are primarily to eliminate the deviation in primary position and abnormal head posture as well as to center the diplopia-free field and to reduce up and downshoots. The secondary goals are to reduce globe retraction and to improve ductions and enlarge binocular field of single vision.

The treatment options are recession of the appropriate horizontal rectus muscle of the affected and/or the sound eye, recession of both horizontal recti (more in one muscle), transposition of vertical recti, and LR inactivation by periosteal fixation.

Recession of one horizontal rectus muscle of the affected eye

Advantages

Decrease in the deviation in primary position

Decrease of abnormal head posture.

Disadvantages

Possible limitation of adduction

Possible decrease of binocular field of single vision

No significant improvement on abduction

No decrease in globe retraction

No decrease in up/downshoot.

Indications

Horizontal deviation

Minimal or no globe retraction

No up/downshoot.

This is the most simple way of management of DS with favorable results; however, it also has some limitations and problems.[29],[30],[31],[32] As most of the patients with DS have some degree of limitation of adduction that may only be identified by a small exotropia on adduction; MR recession causes some degree of limitation of adduction. The decrease of adduction may cause a decrease in binocular field of single vision that may be bothersome for some patients. There is no reliable surgical dose/effect relation for single muscle recession, and the major pitfall is the decision on the amount of recession. If the recession is too much the paradoxical contraction of LR may dominate over the MR contraction, and iatrogenic synergistic divergence may develop in postoperative period.[32] The amount of recession must be determined upon the forced duction test during surgery, and it must be the amount that allows free passive movement of the eye. MR muscle is usually tight, and recession of tight muscles has larger effect on deviation.

If the deviation is large, recession of the appropriate horizontal rectus muscle in the contralateral eye may be considered.[30],[33],[34] Surgery in the yoke muscle of the sound eye must be regarded with caution. The amount of paradoxical contraction of the LR or the paradoxical contraction of vertical rectus muscles may cause unexpected motility results.[35],[36]

Recession of both horizontal recti of the affected eye (more in one muscle)

Recession of both horizontal recti in the affected eye has the advantage of combination with Y splitting of LR muscle where necessary.[37],[38],[39],[40],[41] In our clinical practice, this is the most commonly used procedure in DS [Figure 2]a and [Figure 2]b. Our results demonstrated that recession of both horizontal recti may decrease the globe retraction in 75% of the cases and up/downshoots as well as the deviation in primary position and abnormal head posture [Figure 3].[41] In our series, we concluded that transposition surgery should be kept for those with minimal globe retraction.[41]

Figure 2: Right type 2 Duane syndrome with up and downshoot and significant globe retraction. (b) Right type 2 after recession of both horizontal recti and Y splitting of lateral rectus muscle. Note the decrease of globe retraction as well as up and downshoot

In both single muscle recession and both horizontal rectus recessions, forced duction test must be repeated after disinsertion of the medial or LR muscle. There may be some accessory tissue bands that restrict the ocular movements. Gobin [42] reported the incidence of such bands as 34.3% in cases with DS during surgery. The excision of the tissue band may allow free forced duction test [Figure 4]a,[Figure 4]b,[Figure 4]c,[Figure 4]d.

Figure 4: (a) Bilateral type 1 Duane syndrome with esodeviation and severe limitation of abduction. There is mild globe retraction on adduction in both eyes. (b) Bilateral type 1 Duane syndrome with globe retraction on adduction in both eyes. (c) Bilateral type 1 Duane syndrome. Traction test is positive in both eyes (+4 in the right eye and +3 in the left eye). Traction test is still positive after disinsertion of medial rectus muscle, and a tissue band was recognized behind the insertion beneath the medial rectus muscle on both sides. The tissue band is excised during surgery. (d) In primary position, the patient is orthophoric. Note the decrease of adduction on both sides following medial rectus recession and the tissue band excision

The general rule is to avoid resections in DS. However in eso-DS with minimal globe retraction, recess-resect surgery is reported to be successful if the resection is kept within 3.5 mm at most.[43] In our hands, resection of MR muscle may give good results in Type 2 DS with insignificant limitation of adduction. It must be kept in mind that these are very selected cases and the forced duction test must be negative for the muscle that resection is considered. Even a small resection in a paradoxically contracting and fibrotic extraocular muscle may cause unexpected results, and it should better be used by adjustable sutures where possible.

Cases with horizontal deviations and with minimal or no globe retraction

Severe limitation of abduction

No globe retraction on up and downgaze

No up/downshoot.

Rosenbaum [44] reported that vertical rectus transposition temporally has a greater chance of causing an induced vertical deviation. Augmentation sutures increase the tightening effect.[45],[46] Vertical deviations and increase in co-contraction are reported between 8.5% and 30%, and consecutive exotropia rate is reported as 25.5%.[47]

Transposition of SR instead of two vertical rectus transpositions may be performed with or without MR recession.[48],[49],[50] Augmentation sutures are also possible with this technique. SR transposition is proposed to have a similar effect with both vertical recti transposition. The additional potential problem may be induced incyclotorsion.

Lateral rectus periosteal fixation into the lateral orbital wall

Advantages

Eliminates the effect of paradoxical contraction of LR

Very effective to reduce up/downshoot

Reduces globe retraction.

Disadvantages

Usually requires a combination with vertical rectus transposition surgery

Anterior segment ischemia risk because of transposition surgery

May require secondary surgery

Limited literature data

Technically difficult procedure.

Indications

Severe disfiguring up/down shoot

Synergistic divergence.

LR periosteal fixation is the most recently described method of treatment for DS [51],[52],[53],[54],[55],[56],[57][Figure 5]. The idea is to convert DS into 6th nerve palsy by eliminating the junction of the LR muscle with the globe, so it requires a combination with transposition surgery. LR periosteal fixation eliminates the effect of paradoxical contraction of LR, and it is reported to be very effective to reduce up/downshoots as well as globe retraction. Posterior Tenon or lateral canthal tendon fixation of LR muscle are the proposed alternatives to periosteal fixation.[58],[59] In synergistic divergence which is the most severe form of exo-DS cases with weak MR function, transposition surgery may be considered as a secondary operation where necessary.[56] In our cases, augmented transpositions caused residual exodeviation and nonaugmented transposition gave better results in exo-DS.[55] In eso-DS, secondary MR recession may be required on a separate session as the fourth rectus muscle. In such cases, the major disadvantage of this technique is the anterior segment ischemia risk despite all the measures such as ciliary artery preserving surgery and partial transposition procedures. In our cases, the up/downshoots significantly reduced in all patients whereas globe retraction decreased but not disappeared. The persistence of globe retraction may be due to the possible subclinical paradoxical contraction in transposed vertical recti or LR muscle may still exert a pulling force through the surrounding soft tissues.

The persistence of globe retraction and residual exodeviations demonstrated that LR orbital wall fixation could not convert the patient into a 6th nerve palsy, and the response of surgery is different than that of 6th nerve palsy.

Methods to reduce up/downshoots

The recommended methods which have already been discussed above for treatment of up and downshoots are recession of both horizontal recti, Y splitting of LR muscle, faden operation of both horizontal recti, vertical muscle recession for innervational up and downshoots, and LR inactivation by periosteal fixation.[37],[38],[39],[40],[41],[51],[52],[53],[54],[55],[60],[61] “Y” splitting of LR increases the stability of the LR muscle on adduction. Splitting the LR muscle creates some resection effect and should better be combined with a small recession to compensate this.

Treatment of “Y” pattern deviation

In the atypical DS “Y” pattern deviation, surgical treatment is usually not necessary. Pseudo IO overaction does not respond to IO weakening and supraplacement and recession of LR muscle is demonstrated to be effective.[22],[23]

Role of botulinum toxin A

Botulinum toxin A (BTXA) may be used for diagnostic purpose in DS to demonstrate the possible postoperative result, and in 53% of cases, long-term reduction of the deviation was obtained out of 88 patients.[62]Young patients may also benefit with BTXA injection and 50% success was reported in a series of eight patients.[63]

Conclusions

During preoperative assessment, the deviation in primary position, abnormal head posture, severity of globe retraction, and presence of up and downshoots must be carefully evaluated for a correct surgical plan for the individual patient.

The traction test results are extremely important during steps of surgery.

The surgeon should consider the individual variables and should keep in mind that the vertical rectus muscles may also have some subclinical dysinnervation problem. Although “cure” is not possible, a satisfactory outcome may usually be achieved.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.